It is important to be able to measure the concentration of species such as molecules or ions in a fluid in a range of technical fields, including for environmental, health and industrial applications. There is, therefore, a strong demand for reliable, low-cost and long lifetime sensor technologies which can be used to measure the concentration of species such as chemical molecules and ions in fluids.
For example, there is a significant demand for sensors to measure the concentration of the nitrate ion (NO3−) in drinking water and waste water. The nitrate ion is suspected of being harmful to humans and animals and there are limits placed on the concentration of nitrate ions found in potable water, such as 10 mg/liter nitrate ion as nitrogen (NO3−—N) concentration in the United States and 50 mg/liter nitrate ion (NO3−) concentration in Europe. It is important to be able to confirm that the nitrate ion concentration in drinking water is below these limits, especially when reservoirs, rivers and ground water may be contaminated by nitrate ions from fertilisers used in agriculture. Wastewater with a high nitrate ion concentration which is discharged into the environment can cause eutrophication, such as algal blooms, resulting in oxygen depletion (hypoxia) in the water and having a negative impact on the ecosystem. There are typically limits in place on the maximum concentrations of nitrate ions in water which is released back into the water cycle, and therefore there is a demand for effective nitrate ion sensor technologies to analyse this water.
A growing area of interest in ion sensing is in the food production industries of aquaculture and hydroponics. In aquaculture excessive levels of ammonium ions, nitrite ions or nitrate ions can have an adverse effect on the growth of fish leading to a decrease in yield. In hydroponics the concentration of ions, particularly the nitrate, phosphate and potassium ions, in the feed stock supplied to the crops must be maintained at an optimum level in order to maximise yields. The current prevailing method in both these industries is to send water samples for laboratory analysis to determine the ion concentrations. This entails waiting for the analysis to be conducted and the results returned, meaning that monitoring is not continuous and immediate adjustment of an ion concentration towards a target value is not always possible.
Technologies to address the need for the continuous monitoring of ions are known in the prior art, including ion-selective electrodes (ISEs) and optical sensors. In the case of nitrate ion sensing, the optical approach may be preferable because ISEs suffer from drift, require frequent recalibration and have relatively short lifetimes.
Existing optical nitrate ion sensors rely on the direct absorption of ultraviolet light by the nitrate ion, typically light with a central wavelength less than 240 nm. The concentration of nitrate ions can then be calculated from the measured transmission of light through the sample and the well-known Beer-Lambert law: A=ε·c·L where A is the absorbance given by
      A    =          -                        log          10                ⁡                  (                                    transmitted              ⁢                                                          ⁢              light              ⁢                                                          ⁢              power                                      initial              ⁢                                                          ⁢              light              ⁢                                                          ⁢              power                                )                      ,ε is the molar absorption coefficient of the nitrate ion, c is the concentration of the nitrate ion, and L is the path length through the sample. The nitrate ion molar absorption coefficient spectrum is known in the prior art and is plotted in FIG. 1. The absorption of wavelengths greater than approximately 240 nm is very small and the absorption increases sharply as the wavelength is reduced below 240 nm. In order to generate a wavelength of light which is strongly absorbed by the nitrate ion (i.e. a wavelength less than approximately 240 nm) a xenon lamp or a deuterium lamp is typically used in the optical nitrate ion sensors found in the prior art. Both xenon and deuterium lamps emit broadband radiation (i.e. radiation including a range of wavelengths) which includes ultraviolet radiation with wavelengths less than 240 nm. Other lamps have also been suggested, such as a mercury-iodine lamp which emits at 206 nm.
Numerous features are taught in the prior art which seek to improve the accuracy of nitrate ion concentration measurements which use absorption of ultraviolet light. For example, a reference measurement can be used to determine the intensity of the light source in order to establish the correct value for “initial light power” in the Beer-Lambert equation. Ways to do this include splitting the beam so that a reference channel is created (e.g. DE4407332C2), spreading the beam so that a portion of the beam does not pass through the analyte and combining with a moveable beam block to obstruct either the measurement path or the reference path (e.g. AT408488B), or inserting and removing a reference material into the beam path (e.g. U.S. Pat. No. 3,853,407, WO03067228A1).
The measurement of the absorption by nitrate ions at multiple wavelengths is also detailed in the prior art. For example, the use of two or more wavelengths of ultraviolet light to determine the dependence of absorption on wavelength is described in DE3324606C2 and JP4109596B2.
If the nitrate ion concentration in a fluid is determined from the absorption of light with wavelength in the range 200 nm-240 nm, the measurement may be inaccurate if other components in the fluid (in addition to nitrate ions) also absorb the same light by an unknown amount. This inaccuracy may be reduced using a second absorption measurement at a different wavelength. In one example, patents including GB2269895B, JP3335776B2 and DE10228929A1 disclose the use of an absorption measurement in the wavelength range 250 nm-300 nm to compensate for absorption caused by organic molecules. In a second example, methods of accounting for light scatter caused by suspended particles (turbidity) are detailed in patents DE19902396C2, JP4109596B2 and JP3335776B2, which use transmission at 830 nm, transmission at 633 nm and a direct measurement of the scattered light respectively.
A nitrate ion concentration measurement using two different path lengths to improve accuracy is included in patents DE19902396C2 and AT408488B. The use of a variable measurement path length is included in GB2269895B. This can be used to obtain a preferred strength of the absorption for a particular nitrate ion concentration and thereby extend the measurable concentration range of the sensor. Additionally, patent application JP2000206039A instructs that a longer wavelength should be used for the absorption measurement when high concentrations of the nitrate ion are present.
Two different detector configurations for nitrate ion sensors are described in the prior art. In a first configuration the broadband light from a UV lamp (e.g. xenon lamp or deuterium lamp) propagates through the analyte water; the light which propagates through the water is then filtered using a bandpass filter which transmits light with a range of wavelengths distributed around a central wavelength; and the light which propagates through the filter is then detected using a photodetector (e.g. DE3324606C2). Bandpass filters for suitable deep UV wavelengths (200 nm-240 nm) have relatively poor performance and high cost. For example, commercially available filters with a transmission bandpass full width at half maximum (FWHM) of 10 nm have a maximum transmission of less than 20%. A further disadvantage is that matched filters may be necessary where a reference channel and a measurement channel which operate at the same wavelength are used. In a second configuration the broadband light from a UV lamp propagates through the analyte water; the light which propagates through the water is then detected using a spectrometer which determines the spectrum of the transmitted light (i.e. the intensity of light as a function of wavelength) (e.g. AT408488B). This second configuration can provide high accuracy nitrate ion concentration measurement but spectrometer components have high cost.
The prior art further includes nitrate ion sensors which are either immersion sensors or in-line sensors. An immersion sensor is one where the analyte water to be measured is supplied to the sensor by immersing part or all of the sensor in the water. An in-line sensor is one where the analyte water is continuously supplied to the sensor and the water is measured as it flows between an inlet and an outlet.
The prior art includes devices for optical sensors in other fields (other than for sensing nitrate ions) which use light sources other than xenon and deuterium lamps. Patent application DE102011081317A1 presents an alternative light source for in-line sensor applications. One or more solid-state ultraviolet sources are held in a housing and emit in the wavelength range 240 nm to 400 nm and may have a FWHM within the range 10 nm-20 nm. A method of driving multiple sources by pulsing them in turn to obtain measurements is included.
Another example of the use of a solid-state light source for sensor applications is patent application US20130015362A1 which discloses the use of a frequency-doubled laser as the light source of a sensor for detecting the presence of particles such as bacteria. Neither DE102011081317A1 nor US20130015362A1 provide a device suitable for measuring the concentration of nitrate ions in water. Optical nitrate ion sensors using solid-state light sources have not been found in the prior art.
In summary, optical nitrate ion sensors in the prior art suffer from several disadvantages. For example, the use of deuterium or xenon lamps, which are complex gas-filled light sources, results in nitrate ion sensors which have high cost, occupy large volumes (≥2,000 cm3), require high voltage (≥400 V) driving electronics, have high power consumption (between 2 W and 7.5 W depending on the exact source and configuration used), require warmup-time before a measurement may be made and necessarily include inefficient and high cost bandpass filters or a high cost spectrometer. These disadvantages present a significant barrier to a much more widespread deployment of nitrate ion sensing.